The use of the Exploratory/Developmental Grant mechanism in this study was chosen to allow for the development of assessment tools for determining male responsiveness to infants. The common marmoset monkey, Callithrix jacchus, is the perfect model for studying hormonal activation of male parental care due to father's variation in infant responsiveness, as is found in human fathers. A set of behavioral tests will be used to assess each father's responsiveness to his own infants or to others. These tools will allow us to investigate the role of hormones in modulating male parental care. Specific Aim 1 will explore different methods for evaluating male responsiveness to infants both by his natural performance during the first two weeks following birth and by using responsiveness tests with their own or an unknown infant's vocalizations and their own infant or an unknown infant at three to four weeks of age outside the family environment. The highest and lowest responsive males will be selected for Specific Aim 2. For the second aim, levels of prolactin will be modulated to test changing males' responsiveness towards infants by changing prolactin levels. Males with the highest response towards infants will be given cabergoline to lower their prolactin levels prior to and after birth of their infants. They will be tested with their own and novel infants using the tests designed in Specific Aim 1. Fathers with the lowest response towards infants and nonfathers will be given exogenous prolactin to enhance their infant responsiveness. Specific Aim 3 develops novel techniques for providing female pregnancy cues to males. This aim examines how paternal care is primed by female characteristics. Males will be given scent secretions from pregnant females and tested for their changes in response to infants. Marmosets are one of the few mammalian species whose social organization/family structure bears some resemblance to that of humans. With the development of parenting behavioral assays, the marmoset has the potential to be an ideal model system for studying parental care in primates and providing insights into the human condition. [unreadable] [unreadable]